1. Field of the Invention
The present invention relates generally to an apparatus and process of inspecting printed material. Particularly, this invention relates to an apparatus and process of inspecting printed material by comparing a target image to one or more reference images. More particularly, this invention relates to an apparatus and process of inspecting printed material by comparing the intensity of each pixel in a target image to the average intensity according to the classification of each pixel in the reference image or images. Even more particularly, this invention relates to an apparatus and process of inspecting printed material capable of comparing the intensity of each pixel in a target image to the average intensity according to the classification of each pixel in a reference image or images on a web running at speeds as high as 1200 feet per minute while maintaining a high defect-detection sensitivity and relatively low computational cost.
2. Description of the Prior Art
Through the years devices and methods of inspecting print material have been developed. Several of these devices and methods of inspecting print material has been accomplished by comparing a target image to a reference image including a means for compensating the printing variation that exists during a print run. It is well known in the prior art that the flaw pattern of printed material can be detected by comparing the digital image representation of a target pattern with the digital image representation of a known good sample of such a pattern stored in memory. The digital image representations are generated by digitizing the signal of an optical pickup device such as CCD camera, all as is well known in the art.
The image comparison is accomplished by means of subtracting the digitized intensity value for each pixel in the target image from the intensity value of the corresponding pixel in the reference image. When the difference in the subtracted intensity of the pixels is above the tolerance limit, the pixel is said to contain a flawed pattern.
In reality, however, when the normal printing process variation is introduced, the digitized intensity of the pixels varies more significantly at the spatial location where the process variation occurs, than at other pixel locations in the rest of the image. The intensity tolerance of these affected pixels must therefore be increased to compensate for process variation. Otherwise, a false defect will be detected. An attempt to use a single tolerance for all pixels in the image will cause the entire image to be unnecessarily desensitized, rather than only the affected pixels.
In the multicolor printing press, the impression image of each color ink is transferred to the target material in separate processes. The printing process variation occurs when the web tension varies, the feeding velocity changes, the ink is replenished, the ambient temperature changes, etc. It is obvious that when the printing variation causes the ink impression on the printed material to be displaced, the digitized intensity values of the pixels that are spatially located at the edge of the printing pattern diverge more severely from their ideal reference intensity than the pixels that are located in the inner printing pattern or at the substrate area. Furthermore, the pixel intensities in the inner printing pattern of uniform ink density varies less than the pixels in the area where the ink gradation occurs, such as in the photograph area. The pixel intensity difference due to these acceptable printing variations must be differentiated from the pixel intensity difference due to the actual printing defect before an accurate inspection can be achieved.
Several devices and methods of inspecting printed material on a printing press using the reference image comparison process including compensating for the printing variation, have been disclosed in the prior art. However, those methods have severe limitations. They suffer from reduced sensitivity in the overall defect detection. This is caused by grouping the pixels into units of predetermined areas and determining an average intensity for the group. Groups of the target image are then compared to the corresponding groups of the reference image data. This grouping into units of predetermined areas is necessary because comparison of tone change recognition alone, i.e. intensity, on a pixel-by-pixel basis requires a significant amount of computational power in order to prevent false tone change recognition. An acceptable flaw detection sensitivity for a normally large area printing web requires a large number of digitized pixel elements across the printing image. Furthermore, on web printing systems, the web is run at speeds as high as 1200 feet per minute which require high data processing throughput.
U.S. Pat. No. 5,625,703 (1997, Okuda et al.) teaches a method and apparatus for detecting a defective printed matter in a printing press. Each pixel data of a printed matter is input as test image data. A change in tone, i.e. intensity, is recognized by accumulating and comparing the reference image data and the test image data in units of predetermined areas.
U.S. Pat. No. 5,144,566 (1992, Anderson et al.) teaches a print inspection method in which an area of printed material is optically scanned to obtain image data representing picture elements having variable intensity levels. This is achieved by counting the number of picture elements at a particular intensity level in the area scanned to thereby generate a frequency distribution of the intensity level of the image data in the area scanned. The frequency distribution generated is compared to a stored reference frequency distribution of intensity levels of the image data. A statistical comparison with the reference data is utilized to determine whether or not the printed material is satisfactory.
Therefore what is needed is an apparatus and process for inspecting print matter by comparing a target image to one or more reference images based on the classification and on the average intensity of each pixel. What is further needed is an apparatus and process of inspecting printed material capable of comparing the intensity according to the classification of each pixel of a target image to a reference image data on a web running at speeds as high as 1200 feet per minute while maintaining a high defect-detection sensitivity and a relatively low computational cost. What is still further needed is an apparatus and process of inspecting printed material by comparing the intensity according to the classification of each pixel while tolerating printing variations caused by web error of the printed matter.